1. Field of the Invention
Embodiments of the present invention generally relate to wireless local area networks and more specifically to detecting radar signals within wireless local area networks.
2. Description of the Related Art
Wireless Local Area Network (WLAN) devices must coexist with radar signals in the 5 GHz frequency bands. The general requirement is that WLAN devices should avoid operating on frequencies where radar signals have been detected.
A frequency band may be divided into one or more channels. The bands and channels for one form of wireless communication may be defined by, for example, the IEEE 802.11 family of standards. A WLAN transmitter typically transmits data through a channel to one or more WLAN receivers. A typical bandwidth of a channel is 20 MHz. Moreover, the IEEE 802.11 family of standards may also define how the data may be configured into data packets that typically include a preamble and a payload. The preamble may include training fields that typically precede the payload in each data packet. The IEEE 802.11 family of standards also defines modulation schemes such as Orthogonal Frequency-Division Multiplexing (OFDM) that use closely spaced orthogonal sub-carriers to carry the payload. Each orthogonal sub-carrier frequency is typically referred to as a “bin”, data within each bin is typically encoded for OFDM modulation with a Fast Fourier transform (FFT), and the resulting real (I) and imaginary (Q) parts of the FFT are transmitted.
The IEEE 802.11n draft standard describes how a WLAN transmitter may transmit data through two channels instead of a single channel in order to increase the overall effective bandwidth of a channel, i.e. a wider channel may advantageously increase the data transfer rate. The two channels are typically chosen from within a selected band such that they do not overlap and are often referred to as a control channel and an extension channel. As in the single channel case, a preamble containing training fields precedes the payload transmission on both the control and the extension channels. The typical bandwidths of control and extension channels are the same as in the single channel case (20 MHz), which means the combined bandwidth is approximately 40 MHz.
If a radar signal is detected within a channel used for wireless communications, many specifications (e.g. IEEE 802.11 family of standards) require that WLAN devices leave that channel and move to a channel that does not interfere with the radar signals. In the case of a two-channel configuration, radar signals must be detected in both channels. Many radar signals in the 5 GHz spectrum typically include periodic bursts of radar pulses. The bursts typically have a period of about 1 ms and the pulse duration is typically between 1-5 us. There are many known methods to detect such radar signals. For example, one method measures the pulse duration and the pulse burst period and compares that information against many known radar characteristics. Another method simply looks for the presence of a received signal above a threshold during a ten second start-up period. Yet another method compares the amount of power that appears in-band to the power that appears out-of-band.
Some radar signals, however, have different characteristics. For example, other radar signals may have a wider pulse width between 50-100 us, and an additional characteristic in which the frequency of the radar signal varies in time causing the appearance of some radar signals to move across frequency bins over time. These radar signals may appear as a noise spur or other type of signal interference making radar signal detection difficult. Traditional radar detection methods, such as those described above, may not be effective in detecting some radar signals, particularly those that may change in frequency over time.
Radar signals may exist anywhere within a channel, in some instances aligning with the carrier frequency of a selected channel. Typically, when a WLAN signal is brought down to baseband, the energy of the carrier frequency is suppressed since it appears as a DC offset to the baseband signal and, as such, does not have any modulation information. Thus, if a radar signal is aligned to the carrier frequency, then the radar signal may be difficult to detect. In the case of a two channel WLAN, a radar signal may align with the carrier frequency of the combined control/extension channel. The carrier frequency energy is again suppressed as in the single channel case. Thus, the two channel WLAN also has a radar detection problem.
Therefore, what is needed in the art is a method for detecting radar signals, particularly radar signals that may have a frequency that varies in time and radar signals that may align with carrier frequencies. This method should be applicable to both single and multiple channel WLANs.